Photosystem II (PSII) can be considered a model integral membrane protein complex in which photochemistry, electron and proton transport, and catalysis are facilitated and regulated by multiple polypeptide subunits, multiple ion cofactors and prosthetic groups, and the lipid environment. Our long-term objective is to work toward an understanding of the interplay between all of these factors in the process of water oxidation. In order to realize this objective several lines of research will be pursued. Electron paramagnetic resonance spectroscopic techniques will be used to probe the structure and dynamics of PSII. This work will be directed at characterizing electron donation in PSII and the structure and function of Mn in water oxidation. The spectroscopic studies will be coupled with biochemical manipulations of PSII, such as polypeptide and/or ion depletion and reconstitution. These studies will address the role of specific polypeptides and ions in the structure and dynamics of PSII. Further insight into the mode of action of Mn, Ca(2+), Cl-, and polypeptides in PSII will lead to a clearer picture of the process of water oxidation. Studies of the Mn center in PSII have direct application to the structure and function of other multicenter metalloproteins, such as iron-sulfur proteins; the insight gained on the mechanism of water oxidation by PSII bears on the mechanisms of all enzymes that utilize dioxygen or hydrogen peroxide as a substrate, such as cytochrome oxidase, cytochrome P450, peroxidases, and oxygenases. These studies will also provide important data on the structure/function relations in integral membrane proteins, in general.